Blood and other body samples are frequently collected in small vials or test tubes for subsequent testing. Blood is also collected during blood donation in donation bags. The vials, tubes or bags are sealed to prevent contamination. Testing of the blood typically involves removing, in a sterile manner, a small amount of the blood from the relevant container and, typically, placing a drop onto each of a number of test media. The media may be absorbent paper, a slide or any suitable media. In addition blood in blood donation bags is tested for bacterial contamination. This typically involves placing a small amount of blood from the donation bag in a container having a liquid medium in which bacteria may grow. After some time some of the medium is removed and placed on test media. These containers are commonly referred to as culture bottles.
One common collection vial used is the evacuated tube. Such a tube is, in effect, a sample tube with its open end sealed by a rubber membrane. The tube is supplied at a low pressure and blood or other fluids are introduced by passing a sterile cannula or the like connected to a source of blood (such as a patient, blood donor or blood bag) through the membrane into the interior of the tube. The low pressure assists in drawing blood into the tube. Culture bottles are similar, albeit larger and not being evacuated, in being rigid and having a pierceable membrane sealing an opening.
Removal of a sample of fluid typically involves passing a cannula through the membrane and inverting the vial or bottle. The fluid may be draw out of the container using gravity alone or by connecting the cannula to a syringe and using the syringe to draw the fluid out. Because the vials and bottles are usually rigid, removal of fluid reduces the pressure within and tends to inhibit removal of fluid.
A typical cannula used may be a simple syringe needle having a hollow metal shaft with a pointed end. The interior diameter of the needle is relatively small (about 0.4 mm). A drop of fluid from such a bore typically forms a circle of about 5 to 7 mm diameter when dropped onto typical test media, such as absorbent paper.
Because the bore has a small diameter the amount of fluid required to fill the needle is small and so filling the needle does not reduce the pressure within the vial to a significant amount. This results in the ability to easily remove the necessary number of fluid drops and the fluid flows easily and gives drops of consistent size.
Due to the risk of needle stick injuries the use of metal needles and the like is discouraged.
Cannulas formed of plastics or other polymers have been proposed for other uses. Whilst these have a pointed end, due to the nature of the plastics material they are not considered to be a needle stick risk. However, cannulas formed of plastics cannot be manufactured in a cost effective manner with a passageway with a small diameter corresponding to that of a metal cannula and, typically, are manufactured with a bore diameter of 1 mm or greater. This results in a significantly greater volume of the bore. Attempting to use a plastics cannula with such a large diameter bore to withdraw fluid from a sealed container is not successful and only one or two drops can be obtained before the reduced pressure caused by withdrawing fluid to fill the cannula prevents or limits further withdrawal. In addition, a drop of fluid from a bore of such a cannula is typically unnecessarily large.